Two-parameter kinematic theory for punching shear in reinforced concrete slabs

As a consequence of the dangerous nature of punching failures, the punching shear behavior of reinforced concrete slabs has been in the focus of research for more than 100 years. Due to the complex interaction between bending moments and shear forces in the vicinity of slab-column connections, most of the earlier punching shear resistance models were derived in a “semi-empirical” manner by regressional analysis of the available test data. Subsequently, more general models with different theoretical backgrounds have been developed. For example, models based on kinematic failure mechanisms have been found to be in good accordance with punching tests on slender slabs. The existing kinematic models generally determine the punching strength based on suitable failure criteria relating punching failure to a certain slab rotation. Hence, slab deformations are assumed to occur as a result of flexural deformations only. Yet, measurements taken from recent punching tests with varying slenderness reveal differences between fracture kinematics of slender slabs (e.g. flat slabs) and compact slabs (e.g. column bases). In this context, the deformation behavior of compact slabs is rather dominated by translational deformations. Consequently, a general application of the existing models to both slender and compact slabs might yield inconsistent results. In the present thesis, the punching shear behavior of reinforced concrete flat slabs and column bases is investigated in detail. Based on measurements and theoretical investigations, the fracture kinematics of slabs failing in punching are analyzed. The investigations verify that the total deformation of compact slabs at punching failure is significantly underestimated by considering the slab rotation as single degree of freedom (DOF). A more general description of the deformation behavior of both slender and compact slabs is possible by introducing a second DOF considering translational deformations. Based on the aforementioned observations, a two-parameter kinematic theory for punching shear in reinforced concrete slabs without shear reinforcement is developed. In the theory, it is assumed that shear forces are transmitted along the failure crack by four shear contributions, namely the contributions of compression ring, aggregate interlock, residual tensile stresses, and dowel action. The magnitude of shear contributions is estimated based on the deformed slab accounting for both DOFs. Subsequently, the punching strength is calculated by summation of the contributions. The evaluation of the proposed theory by means of systematic test series and databanks yields good agreement between predictions and experimental results. Especially, the differences between flat slabs and column bases can be explained in a consistent manner by the theory. To investigate the generality of the proposed two-parameter kinematic theory, further investigations are carried out to extend the theory to other cases, such as prestressed slabs, continuous slabs, and shear-reinforced slabs. The investigations verify that it is possible to account for the beneficial effects of prestressing, slab continuity, and shear reinforcement on punching strength by means of the proposed kinematic theory. Moreover, good accordance between predictions and results of punching tests investigating the aforementioned influences is found